BACnet is "a data communication protocol for building automation and control networks."

A data communication protocol is a set of rules governing the exchange of data over a computer network. The rules take the form of a written specification (in BACnet's case they are also on CD) that spells out what is required to conform to the protocol.

How can BACnet do things in a standard way?

The trick is that BACnet provides a standard way of representing the functions of any device, as long as it has these functions. Examples are analog and binary inputs and outputs, schedules, control loops, and alarms. This standardized model of a device represents these common functions as collections of related information called "objects," each of which has a set of "properties" that further describe it. Each analog input, for instance, is represented by a BACnet "analog input object" which has a set of standard properties like present value, sensor type, location, alarm limits, and so on. Some of these properties are required while others are optional. One of the object's most important properties is its identifier, a sort of numerical name that allows BACnet to unambiguously access it. Once devices have common "appearances" on the network in terms of their objects and properties, it's easy to envision messages that can manipulate this information in a standard way.

Is BACnet limited only to HVAC equipment? Can it be used with fire/life safety, lighting control, and other building automation systems?

Absolutely. In fact, if you think about it, BACnet already contains most of the capabilities required for non-HVAC communications. These include the ability to read and write binary, analog, and text data; schedule control actions; send event and alarm notifications; and so on. Nonetheless, the BACnet committee realized that these capabilities might not cover all situations and developed the standard with an eye toward accommodating future, unknown building automation and control applications. As a result, one of the real strengths of the BACnet model that emerged from this consideration is that it can be easily extended. If a vendor comes up with some new functionality for which communication is required, the vendor can add new properties to existing object types or create new object types that are accessed in exactly the same way as the eighteen defined in the standard. This is not only expected, it is encouraged. Moreover, a vendor could even dream up new services that go beyond the standard ones. Of course, proprietary features may not be interoperable without vendor cooperation.

What networking options are there for BACnet?

The BACnet committee spent a lot of time on this part of the standard. 5 different options were created, each of which fills a particular niche in terms of the price/performance tradeoff. The first is Ethernet, the fastest at 10 Mbps with 100 Mbps also recently available. ("Mbps" stands for "millions of bits per second.") Ethernet is also likely to be the most expensive in terms of cost per device. Next comes ARCNET at 2.5 Mbps. Both Ethernet and ARCNET can use a variety of physical media ñ coaxial cable, twisted pairs, even fiber optic cable. For devices with lower requirements in terms of speed, BACnet defines the MS/TP (master-slave/token-passing) network designed to run at speeds of 1 Mbps or less over twisted pair wiring. Echelon's LonTalk network can also be used on various media. All of these networks are examples of "local area networks" or LANs. BACnet also defines a dial-up or "point-to-point" protocol called PTP for use over phone lines or hardwired EIA-232 connections. A key point is that BACnet messages can, in principle, be transported by any network technology, if and when it becomes cost-effective to do so.

Can any equipment that uses LonTalk can automatically talk to BACnet systems?

Unfortunately not. LonTalk is Echelon's specification for a LAN technology that many people thought would be a useful addition to the BACnet standard. BACnet uses LonTalk to convey BACnet messages in an identical manner to the way BACnet messages are transported by Ethernet, ARCNET, and MS/TP. Confusion stems from the fact that Echelon has its own control language that is also transported by LonTalk. The point is that the BACnet language and the Echelon language are fundamentally different and devices using one of the languages can never interoperate directly with devices using the other, even though they might possibly share a common LonTalk LAN.

 

For more information about BACnet, please visit Bacnet.org, HPAC, or the ASHRAE Journal